The invention resides in an electric universal drive based on a piezo motor which is particularly designed for use as a vehicle drive but may also be used for various other drive applications. It is described below in the form of a piezo motor universal drive. But the piezo actuators may be replaced by equally or similarly effective high-frequency actuators.
As vehicle drives, in particular of motor vehicles, the drive according to the invention may be effective via a drive element in the form of a disc coupled to a wheel shaft. In a motor vehicle for example, a brake disc may be used as the disc cooperating with the drive according to the invention which disc then is a drive disc as well as a brake disc.
U.S. Pat. No. 7,638,329 discloses a piezoelectric motor for use as a vehicle drive, a control drive or similar. It includes two motors arranged in a mirror-reversed manner at opposite sides of a drive or braking disc. The motors are arranged each between a reference part, that is, a frame or housing and the drive or braking disc forming the driven part.
The motor includes alternately electrically energized clamping piezo packets and two oppositely disposed stepping piezo packets assigned to each clamping piezo packet, which are arranged in longitudinally-spaced relationship from the clamping packet and the driven part and are connected to a leg which is quasi-linked to the clamping piezo packet and which acts on the driven part.
The clamping piezo packets serve in each case for the selective generation of a clamping force between the reference and the driven part which is the reason that they are called clamping piezo packets. These clamping piezo packet are in each case connected with one end of the piezo element stack arrangement to the reference part and are supported at the other end via the leg on the driven or drive part without being connected thereto. The clamping piezo packets are arranged each side-by-side in rows wherein the orientation of the row is transverse to the relative movement between the driven part and the reference part.
Each of the two mirror-reversed arranged drive arrangements has two groups of clamping piezo packets and two groups of stepping piezo packets. They are electrically so controlled that, while the one group of clamping piezo packets is activated, that is, electrically energized for an expansion thereof, and these clamping packets engage via the leg connected thereto the driven part, the other of the group of clamping packets is de-activated that is it is in the release position, in which it is not in engagement with the driven part. Then the group of stepping piezo packets assigned to the energized clamping piezo packets is activated so that the respective stepping piezo packets expand and, in this way, displace by a certain distance the legs of the clamping piezo packets in engagement with the driven part. As a result, the driven part is moved by a corresponding distance past the clamping piezo packets of the other clamping piezo packet group which is in the release position. Subsequently, the control is switched over, that is, the previously clamping piezo packets are switched to a release position together with the associated stepping piezo packets, so that the previously clamping piezo packets are freed from the clamping position and return to their rest position while, at the same time, the other group of clamping piezo packets which previously were in the release position are energized that is switched to their clamping position whereupon also their stepping piezo packets are activated. In this way, the two groups of clamping piezo packets with the respective associated stepping piezo packets provide for a continuous stepping sequence by their alternating energization whereby the driven part is moved in a stepwise fashion. Since this control of the piezo packets occurs in the frequency range of several kilohertz, for example 25 kHz, a given continuous movement of the driven part relative to the reference part is obtained.
For the cooperation between the clamping piezo packets and the driven part to function, the clamping piezo packets between the reference part and the driven part must be held under tight pretension since otherwise, with the available expansion stroke of the clamping piezo packets, the required clamping force and consequently the necessary friction force for the movement of the driven part cannot be reached.
It is known already from U.S. Pat. No. 7,638,329 that, to this end, the clamping piezo packets arranged in a row are supported in each case via a so-called semi-rigid bridge on the reference part under pre-tension. In this arrangement, the driven part needs to be rigid. In connection with the “Semi-rigid” bridge, the term “semi-rigid” means that the bridge adapts at least in the longitudinal tolerance and the expansion stroke range of the clamping packets in the static state to stack height differences of the clamping piezo packets by elastic bending, but in the dynamic state remains essentially rigid with the operating frequency. This is based on the recognition that the movement of piezo packets is very small. The available expansion length of a piezo packet is about one thousandth of the stack height.
In the piezo-electric motor according to U.S. Pat. No. 7,638,929, the content of which is enclosed herewith by reference, it is proposed for an improvement of the principle of the “semi-rigid bridge” to provide it in the form of separate frictionally joined bridge elements which are assigned in each case to a clamping piezo packet. It is also provided that, by a selection of the geometry of the arrangement of the stepping piezo packets, specifically the positioning of the arrangement of the stepping piezo packets relative to each other in an opposite sense, an optimally adapted “transmission ratio” between the stepping piezo expansion of the leg movement at the connecting point of the legs to the driven part can be adjusted. In this way, with an appropriate operating frequency of the piezo packets in an application as a motor vehicle drive with direct transmission to the wheel shaft for example via a brake disc serving as a drive disc, speeds of over 200 km/hr can be achieved. In addition, a speed control can be achieved in a simple manner by a phase shifting of the control for the oppositely energized stepping piezo packets.
It is the object of the present invention to provide, based on the use of a piezo-electric motor of the type as known from U.S. Pat. No. 7,638,329, a complete drive arrangement for use for example as vehicle drive, wherein beyond the specific requirements for the piezo motor also the other conditions as they apply specifically during operation of a motor vehicle are taken into consideration.
As explained above with reference to the piezo motor of U.S. Pat. No. 7,638,329, it is important for the functioning thereof that the driven part and the clamping piezo packets are tightly pre-tensioned. During the stepping procedure of the legs of the clamping piezo packets for driving the disc serving as driven part, the machine clamping packet legs are raised from the driven part only minimally that is a few hundredths of a millimeter.
The piezo motor may not only be used as a motor for driving the drive disc but also for braking in that it is driven by the drive disc and in the process operates as an electric generator whereby it resists the rotation of the drive disc. For braking a motor vehicle however, it is necessary to provide additional friction brake pads for generating strong braking effects.
In hybrid drives of motor vehicles with a selectively activatable internal combustion engine and electric motor, it is, in connection with the use of a piezo motor as electric motor, also necessary that the piezo motor can be switched off the drive disc when the vehicle is to be operated by the internal combustion engine. This means that then the legs of the clamping piezo packet all need to be raised off the drive disc in contrast to the tight engagement required for active operation. A release switching arrangement performing this function has also been proposed in U.S. Pat. No. 7,638,329 discussed earlier. However, an improvement with respect to minimal fault sensitivity and unlimited operation would still be desirable.
The object of a further development of the piezo motor as a full drive is to provide all the necessary functions that are needed for example for a motor vehicle drive and have them integrated into a complete vehicle drive, which furthermore also can take over the function of braking.
A vehicle drive on the basis of a piezo motor is desirable in particular also because, in this way, an interesting alternative to the so far commonly used electric motor drives of motor vehicles, especially in the form of hybrid drives is offered. In the electric motor vehicle drives used so far characteristically, there is a need for the use of highly effective magnetic components of rare earth magnets for which the global resources are very limited and which, with heavy utilization in motor vehicle drives, are easily subject to corrosion. In addition, the large amount of necessary copper components of electric motors is problematic.